Process and apparatus perforating tubular plastic foil with a laser beam

ABSTRACT

A continuously conveyed tubular plastic foil is perforated by a laser beam which is focused by a mirror or lens on the perforation forming plane, lying at the focal point of the mirror or lens, of a tautly conveyed tubular plastic foil. For taut conveyance of the tubular foil it is guided over a support placed in the tubular foil, or over supporting plates, lying outside the tubular foil and provided with a foil suction device and with laser beam transmission apertures. The slightly tapering perforations with a minimum aperture at the inside of the tubular foil form, on the one hand, a pattern of parallel lines running in the lengthwise direction and, on the other hand, a pattern of parallel lines which run at an angle relative to the longitudinal axis of the flattened tubular foil. A bag can be formed from the tubular foil. An apparatus for perforation is made up of, in addition to a laser beam source, a lens beam-directing device and a lens or mirror, a support to be placed in a tubular foil, or supporting plates, provided with apertures and with foil suction apertures, for taut conveyance of the tubular foil in a perforation forming plane.

FIELD OF THE INVENTION

The invention relates to a process and an installation of manufacturinga perforated tubular plastic foil through the action of a laser beam.

DESCRIPTION OF PRIOR ART

Such a process for perforating a plastic foil using a laser beam isknown. Plastic foils perforated with a laser beam in fact constitute avery suitable material for the production of bags for packing loosematerial in suitable quantities of, for example, 25 kg. Since in thefilling of the bags with loose powder-type material this loose materialis conveyed with air, after the bag has been filled a quantity of air isleft behind between the particles of the powder material, and this airmust be able to excape while leaving the powder material behind in thebag. Moreover, after filling, these perforations permitting a rapiddeaeration must also be sealed quickly and as well as possible, in orderto prevent loss of material and the penetration of moisture into thebag.

However, when a plastic tubular foil lying in a flat position is beingperforated, it is difficult to obtain perforations of the desired size.If the plastic foil material to be perforated is too thick, a conicalperforation being large at the entry side of the laser beam and small atthe opposite side is produced. This means that in a tubular foil whichis lying flat a relatively large or even too large a perforation isproduced in the top layer of the foil, and a relatively small or toosmall a perforation in the layer of the foil underneath it, which isintegral with the top layer of the foil on which the laser beam actsdirectly and first.

Another disadvantage is that when the tubular foil is lying flat the twolayers of plastic lying closely on top of each other are together ofsuch great thickness that the laser beam is difficult to focus over theentire thickness of the two layers of foil, which means that during theperforation with the laser beam melting of the perforation edges occurs.

In the case of perforating the plastic tubular foil with a layer of airbetween the two layers of plastic of the tubular foil the problem isthat the laser beam is then not focused sharply enough, while the amountof energy for formation of the perforations becomes unacceptably high,and consequently too large perforations are formed and are, moreover,formed in imprecisely defined places.

In order to avoid these shortcomings U.S. Pat. No. 4,254,916 discloses aprocess of manufacturing a perforated tubular foil by means of a laserbeam wherein the wall part of the foil to be perforated is guided in aperforation forming plane by sucking said wall part onto a supportingsurface located outside the tubular foil, the remaining wall part of thetubular foil being held free from the wall part to be perforated.

However, this process presents the disadvantage that often the foilsurface to be perforated is not tautly sucked onto the tubular foilsupporting means situated outside the tubular foil. This involves theformation of creases in the foil surface leading to perforations at theincorrect position and/or with incorrect dimensions.

SUMMARY OF THE INVENTION

The object of the invention is now to provide a process of manufacturinga perforated tubular plastic foil through the action of a laser beam onthe tubular wall of a tubular foil thereby forming perforations rangingfrom 50 to 250 μm in size, and in which perforations of the desired sizeare obtained in the correct plane and with the correct dimensions,without melting of the perforation edges and without inadmissibly highenergy consumption.

Thus, the invention relates to a process of manufacturing a perforatedtubular plastic foil through the action of a laser beam on the tubularfoil wall, forming perforations ranging from 50 to 250 μm in size,wherein during the perforation action the wall part to be perforated andlying free from the remaining wall of the tubular foil is guided in aperforation forming plane on which the laser beam is founded, as knownfrom U.S. Pat. No. 4,254,916, being characterized in that the wall partto be perforated is conveyed tautly over a support placed in theinterior of the tubular foil.

By guiding the wall part of the tubular foil to be perforated tautly ina perforation forming plane according to the process of the invention,the laser beam can now be focused accurately, and an accurately definedperforation can therefore be obtained.

It is particularly advantageous for the laser beam to be focused usingat least one lens and/or mirror on the perforation forming plane whichlies at the focus of the lens and/or mirror.

In this way perforation apertures which meet the required standards inthe optimum way are obtained with the optimum use of energy.

It is expedient if during the perforation action two opposite wall partsof the tubular foil are each perforated in a perforation working plane,so that the perforations can be distributed uniformly over the tubularfoil surface.

In the process according to the invention the tubular foil isadvantageously conveyed continuously, which means that a large number ofperforations can be formed quickly.

An intermittent laser beam expediently moves at right angles to thedirection of conveyance along the continuously moving tubular foil,thereby forming a pattern of perforations placed in parallel lines.

The invention also relates to a plastic bag formed from a perforatedtubular plastic foil by using the process according to the invention.

In particular, the invention also comprises a perforated tubular plasticfoil with perforation apertures ranging from 50 to 250 μm formed bylaser beam, and provided with a pattern of parallel lines (A) ofsuccessive perforations formed by laser beam, while in the transversedirection the perforations form a pattern of parallel lines (B) runningat an angle relative to the longitudinal axis of the tubular foil acrossthe width of the flattened tubular foil, and a bag formed therefrom.

Finally, the invention relates to an installation for carrying out theprocess according to the invention, at least comprising a device forforming a laser beam, tubular plastic foil supporting means, and tubularplastic foil conveying means for conveying the tubular foil.

The installation according to the invention at plastic foil dichargemeans, laser beam action interruption means, tubular plastic foilsupporting means guiding a wall part of a tubular plastic foil to beperforated in a perforation forming plane, while the other part of thetubular plastic foil lies free from this perforation forming plane, andbeam directing means and at least one lens for focusing the laser beamon the above-mentioned perforation forming plane as known from U.S. Pat.No. 4,254,916, which is characterized according to the invention in thatthe supporting means form a support to be placed inside a tubular foil,and over which the tubular foil can be guided tautly.

The installation according to the invention at least has lenses set upin such a way that the focal point of said lens lies in a perforationforming plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with reference to an example of anembodiment as shown in the drawing, in which:

FIG. 1 shows a top view of a tubular plastic foil with perforationsformed by using the process of the invention;

FIG. 2 shows an installation for carrying out the process according tothe invention with a support to be placed inside a tubular foil; and

FIG. 3 shows a plastic bag formed from a perforated tubular plastic foilobtained by the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen in FIG. 1, a tubular foil 1 is provided with perforations2 ranging from 50 to 250 μm in size.

The perforations form a pattern of parallel lines (A) in the lengthwisedirection of the tubular plastic foil 1, and in the transverse directionof the tubular plastic foil the perforations 2 form lines (B) which runat an angle relative to the longitudinal axis of the foil. These lines Aand B are shown as dashed lines.

FIG. 3 shows a plastic bag 3 made from a tubular plastic foil perforatedby the process according to the invention, with a transverse sealforming a bottom 3, and a filling aperture 5 bounded by a peripheraledge 6.

FIG. 2 shows an installation for carrying out the process according tothe invention. This installation comprises a stock reel 7 for tubularplastic foil 1, tubular foil conveying means in the form of feed rollers8 for conveying a tubular foil, tubular plastic foil discharge means inthe form of discharge rollers 9, and a storage reel 10 for storingperforated tubular plastic foil 1. The tubular foil can be conveyedcontinuously by means of the feed rollers 8 and discharge rollers 9.

The installation is also provided with a laser beam source 11 and anumber of lenses 12 placed next to each other, and each set to make thelaser beam act on a perforation forming plane 13, 13' where a tubularfoil wall 1a, 1b is being conveyed tautly. The perforation forming plane13 and 13' is formed by a support 14 placed in the tubular foil 1, withtubular foil wall 1a, 1b being conveyed tautly over this support 14. Thesupport 14 is held fixed by supporting rollers 19.

The above-mentioned perforations 2 are obtained by fitting a number oflenses 12 arranged across the width of the tubular foil, and making thelaser beam act upon the adjacent lenses 12. Through working with severallenses 12 and a moving lens beam-directing means, it is possible to makedo with one laser beam source 11.

Since the intermittent laser beam, which alternately directs a beam onone lens 12 at a time by means of lens beam-directing means 18, isalways focused here by the lens 12 in question on the perforationforming plane 13, 13', perforations of the desired size of 50 to 250 μmare obtained in the correct place.

It is expedient to work with mirrors 18, in particular withbeam-directing means designed with one or more rotatable mirrors 18 inorder to be able to direct the laser beam towards one lens 12 of anumber of adjacent lenses, and subsequently on the perforation formingplanes 13, 13' which always lie at the focal point of the lenses 12. Therotation facilities of the mirrors 18 are shown schematically by thecurve 20. A laser beam emitted by the laser beam source 11 is directedby rotating mirror 18 onto one of the lenses 12 and then by this lens 12onto the perforation forming plane 13 or 13', where the tubular foil isconveyed tautly. The path of a laser beam is indicated by dashed line21.

Although two laser beam sources 11 are shown in the figure, it is alsopossible to work with one source 11.

The pattern of the perforations shown in FIG. 1 is obtained by focusingone laser beam onto a continuously conveyed tubular foil above which aset of lenses 12 is set up at regular intervals from each other over thewidth of the foil web. The laser source works intermittently in such away that the laser beam acts successively on a next lens 12 in thetransverse direction. In view of the conveyance of the tubular foil inthe period of time in which the laser beam acts on the first lens andsubsequently on the lens following in the transverse direction of thefoil, the perforation 2" formed by a following lens 12 will lie lower,as seen in the lengthwise direction, than the perforation 2'. In thefinished foil the perforations 2' to 2'"" consequently lie on animaginary line B which runs at an angle relative to the longitudinalaxis of the tubular foil.

The perforations are preferably made slightly tapered, with a minimumaperture at the inside of the tubular foil.

It is claimed:
 1. Process of manufacturing a perforated tubular plasticfoil having an inside surface and an outside surface defining aninterior through the action of a laser beam on a tubular foil wall,comprising the steps of:forming perforations ranging from 50 to 250 μmin size, wherein during the perforation action a part of the wall to beperforated is guided in a perforation forming plane on which the laserbeam is focused, said part to be perforated lying free from a remainingrest of the wall of the tubular foil; and conveying the wall part to beperforated tautly over an elongate support having front and rear endsplaced within the interior of the tubular foil; holding said support inplace by a plurality of rollers disposed on the outside surface of thetubular foil and located so as to cooperate with the ends of saidsupport.
 2. Process according to claim 1, wherein the perforations aremade slightly tapering with a minimum aperture on the inside surface ofthe tubular foil.
 3. Process according to claim 1, wherein during theperforation action two opposite wall parts of the tubular foil are eachperforated in a separate perforation working plane.
 4. Process accordingto claim 1, wherein the tubular foil is conveyed continuously. 5.Process according to claim 1, wherein the tubular foil is movedcontinuously in a direction of conveyance and wherein further anintermittent laser beam moves at right angles to the direction ofconveyance along the continuously moving tubular foil, thereby forming apattern of perforations placed in parallel lines.
 6. A process ofmanufacturing a perforated tubular plastic foil having an inside surfaceand an outside surface defining an interior through the action of alaser beam on a tubular foil wall, comprising the steps of:formingperforations ranging from 50 to 250 μm in size, said perforations madeslightly tapering with a minimum aperture on the inside surface of thetubular foil, wherein during the perforation action a part of the wallto be perforated is guided in a perforation forming plane on which thelaser beam is focused, said part to be perforated lying free from aremaining rest of the wall of the tubular foil; and continuouslyconveying the wall part to be perforated tautly over a support placed inthe interior of the tubular foil, wherein the tubular foil is movedcontinuously in a direction of conveyance and wherein further anintermittent laser beam moves at right angles to the direction ofconveyance along the continuously moving tubular foil, thereby forming apattern of perforations placed in parallel lines.
 7. Process ofmanufacturing a perforated tubular plastic foil having an inside surfaceand an outside surface defining an interior through the action of alaser beam on a tubular foil wall, comprising the steps of:forming apattern of perforations placed in parallel lines and slightly taperedwith a minimum aperture on the inside surface of the tubular foil andranging from 50 to 250 μm in size by moving an intermittent laser beamat right angles to a direction of conveyance, wherein during theperforation action two opposite wall parts to be perforated are guidedsuch that said opposite wall parts are each perforated in a separateperforation forming plane on which the laser beam is focused, said partto be perforated lying free from a remaining rest of the wall of thetubular foil; and conveying continuously in the direction of conveyancethe wall part to be perforated tautly over an elongate support havingfront and rear ends placed within and held in place in the interior ofthe tubular foil.
 8. Apparatus for perforating tubular foil, comprisinga device forming a laser beam, tubular plastic foil supply means,tubular plastic foil conveying means for conveying the tubular foil andadapted for continuous conveyance, tubular plastic foil discharge means,laser beam action interruption means, tubular plastic foil supportmeans, said support means comprising an elongate supporting memberhaving a front end and a rear end cooperating with a plurality ofsupport rollers located at each end thereof, said support means guidinga wall part of a plastic tubular foil to be perforated in a perforationforming plane, while that part of the tubular plastic foil not beingperforated lies free from this perforation forming plane, andbeam-directing means and at least one lens for focusing the laser beamon said perforation forming plane, the supporting member adapted to beplaced inside said tubular foil, and over which the tubular foil can beguided tautly, with said support rollers disposed outside of saidtubular foil.
 9. Installation according to claim 8, further includingone or more rotating mirrors for directing a laser beam onto the lenses.